In the cases of crankshafts which are used in reciprocating engines of vehicles, a steel material made from carbon steel or low alloy steel is hot-forged to be roughly molded, subjected to a heat treatment such as thermal refining, and then machined into a shape of a crankshaft bent in a longitudinal direction. Furthermore, a surface hardening treatment such as nitrocarburizing is performed. In addition, in the case where a microalloyed steel is used, the heat treatment after the hot forging may be omitted and the machining may be directly performed. Particularly, in the case where a bainitic microalloyed steel is used to give a micro metal structure mainly composed of bainite, the heat treatment such as thermal refining can be omitted by controlling the thermal history during the hot forging, and fatigue strength as crankshafts can be improved.
For example, Patent Document 1 discloses a method of manufacturing a nitrocarburized machine part such as a crankshaft using a bainitic microalloyed steel with omission of a heat treatment after hot forging. By adjusting the component composition of the microalloyed steel and by optimizing a heating temperature before hot forging and a cooling rate after forging, the ratio of a bainite structure in the micro metal structure of the hot-forged product is adjusted to 50% or greater, and the fatigue strength can be increased. In addition, by optimizing nitrocarburizing treatment conditions, a nitrocarburized machine part having: such mechanical properties that it can be subjected to machining such as cutting in terms of industrial production; and high fatigue strength can be obtained even when a heat treatment after hot forging is omitted. In greater detail, by adjusting the component composition of the microalloyed steel, a DI value which is an index of hardenability and a Kp value which is an index of a critical cooling temperature at which pearlite is generated are limited to predetermined ranges, and the carbon equivalents of the contained elements are limited, whereby hardness which is required for machine parts and cutting processability (machinability) are secured, and the fatigue strength can be further improved with the contents of Si and Mn.
In addition, since crankshafts having a shape bent in a longitudinal direction are easily bent during a surface hardening treatment, it may be necessary to perform a bending straightening treatment for straightening the bending after the surface processing. Meanwhile, in general, steel materials having high fatigue strength as a crankshaft have high mechanical strength, and thus it may be difficult to perform such a bending straightening treatment.
For example, Patent Document 2 discloses a method of manufacturing a nitrocarburized crankshaft, in which a bainitic microalloyed steel having Mo and V added thereto is used and a strain releasing heat treatment for releasing processing strain is applied before a nitrocarburizing treatment step to suppress bending occurring by the nitrocarburizing treatment to thus omit a bending straightening treatment after the nitrocarburizing treatment. Such a strain releasing heat treatment is preferably performed for 10 minutes to 180 minutes at 300° C. to 650° C. in consideration of the release of the processing strain and a reduction in strength due to overaging. Furthermore, it is disclosed that by adjusting the contents of C, Si, Mn, Cr, Mo, and V and by limiting a Kf value which is an index of the generation and stabilization of bainite, a Hf value which is an index of hardness of the hot-forged product, and a Hg value which is an index of internal hardness after the nitrocarburizing treatment to predetermined ranges, the ratio of the bainite structure is adjusted to 70% or greater, and by controlling the hardness after hot forging, machinability can be secured and the internal hardness after the nitrocarburizing treatment can be increased.    [Patent Document 1] JP-A-2007-146232    [Patent Document 2] JP-A-2011-094203